The use of on and off tools (or on-off tools) for connecting and disconnecting portions of a rod string is known in the petroleum industry. In the typical application, the on and off tool is used to prevent stripping jobs. A stripping job is required when a downhole pump or plunger cannot be unseated from the tubing string. In such cases, unless the rod string can be otherwise disconnected from the pump or plunger, the tubing string and rod string are pulled together, with the tubing “stripped” over the rod string. However, with an on and off tool, the section of the rod string above the tool may be released by activating the tool, leaving the section of the rod string below the tool in the tubing. In this application, the primary purpose of the on and off tool is the “off” function.
However, in some applications, the primary purpose of the on and off tool is the “on” function, where it is desirable to be able to connect an upper section of rods to a lower section of rods previously installed within a tubing string. For example, if the plunger of a downhole pump has a larger diameter than the drift diameter of the tubing, the plunger assembly having a lower portion of the on-off tool attached, will be installed in the well as the tubing string is installed. Once the tubing string is in place, an upper section of the rod string is run into the well, where the lower most rod of the upper section has the upper portion of the on-off tool attached to it. The upper portion of the tool engages the lower portion, thereby connecting the upper section of the rod string to the lower section of the rod string. When servicing is required, the tool is clutched, rotated, and un-latched so that the rod string can be retrieved to surface while the plunger assembly remains down-hole with the tubing.
The currently known on-off tools have in a small gap between a housing member in the upper portion of the tool and a bushing in the lower portion of the tool. Reciprocating motion during pump operation causes the gap to close and to open every cycle. Under certain well conditions, this phenomenon can cause the on-off tool to prematurely un-latch such that the lower portion of the rod string is separated from the upper portion of the rod string. A premature release of the on-off tool causes downtime and a significant monetary loss because a crane or work-over rig, at significant expense, is required to re-engage the on-off tool. During the period the on-off tool is disengaged well production is lost. Moreover, if this event happens once, it's likely that it will happen repeatedly on the same well. There is currently no known solution to this problem.